Malaria caused by the parasite Plasmodium vivax is a major disease of mankind in the tropics and subtropics causing a most debilitating disease which is characterized by intermittently high fever paroxysms. This leads to great human suffering and huge economic losses in countries afflicted by that form of malaria. Understanding the biology and pathogenicity of this parasite is of great importance particularly in view of future drug and vaccine developments. However, a prerequisite for this is the development of a continuous in vitro culture. A continuous culture system for P. falciparum by Trager and Jensen made that parasite available for extensive in vitro studies. This culture system has been one of the most far-reaching breakthroughs in malaria research and has led to the detection of antigens which permit the development of an anti-malaria vaccine, particularly against the erythrocytic stages of the life cycle of that parasite. Except for P. falciparum, no other human malaria parasite (P. vivax, Plasmodium ovale, Plasmodium malariae) has been continuously cultivated in vitro. In this project, attempt will be made to develop techniques for the continuous in vitro cultivation of P. vivax. The approach will start with cultures utilizing squirrel monkey erythrocytes and serum. Varius media and medium supplements will be used to promote in vitro development of the malarial parasite. Once successful cultivation is achieved with the monkey cells and serum they will be gradually replaced with human erythrocytes and serum. The significance of any in vitro cultivation lies in the fact that it will permit extensive research without the use of animal models. In the case of P. vivax malaria, an in vitro cultivation system will not only conserve precious primates in the future, but will also permit extensive laboratory work which is impractical to do with animal models.